Managing Unlinkable Identifiers for Controlled Privacy-Friendly Data Exchange

1. A method for managing unlinkable database user identifiers, the method comprising: distributing to a first database a first encrypted user identifier, a first database identifier, and a first database user identifier; distributing to a second database a second encrypted user identifier, a second database identifier, and a second database user identifier; receiving from the first database a third encryption and a fourth encryption, the third encryption being formed from the first encrypted user identifier, the second database identifier, and a message comprised in the fourth encryption; decrypting the third encryption thereby obtaining a decrypted value; deriving a blinded user identifier from the decrypted value; sending an encrypted blinded user identifier and a fourth encrypted value to a second server thereby enabling the second server to compute the second database user identifier from an encrypted blinded database user identifier and a decrypted fourth encrypted value; generating a blinded encrypted expression comprising the first database identifier after receiving the first encrypted user identifier; and distributing the first database identifier in a blinded, encrypted form using thereby the first encrypted user identifier, wherein the second encrypted user identifier is generated using an encryption function using a public key of a central authority and a user identity as input and wherein the second database user identifier is generated using a pseudo random function using as input a user identity and the second database identifier, wherein the first encrypted user identifier, the second encrypted user identifier, the first database user identifier and the second database user identifier are deleted after the distribution to the first database and the distribution to the second database, respectively.

2. The method of claim 1 , wherein the first encrypted user identifier is generated using an encryption function using a public key of a central authority and a user identity as input.

3. The method of claim 1 , wherein the first database user identifier is generated using a pseudo random function using as input a user identity and the first database identifier.

4. The method of claim 1 , wherein the distributing to the first database is performed by the central authority and wherein the first database is run on a first server.

5. The method of claim 1 , wherein the distributing to the second database is performed by the central authority and wherein the second database is run on a second server.

6. A system for managing unlinkable database user identifiers, comprising: a hardware processor implemented first distribution unit adapted to distribute to a first database a first encrypted user identifier, a first database identifier, and a first database user identifier; a hardware processor implemented second distribution unit adapted to distribute to a second database a second encrypted user identifier, a second database identifier, and a second database user identifier; a hardware processor implemented receiving unit adapted to receive from the first database a third encryption and a fourth encryption, the third encryption being formed from the first encrypted user identifier, the second database identifier, and a message comprised in the fourth encryption; a hardware processor implemented decrypting unit adapted to decrypt the third encryption thereby obtaining a decrypted value; a hardware processor implemented deriving unit adapted to derive a blinded user identifier from the decrypted value; a hardware processor implemented sending unit adapted to send an encrypted blinded user identifier and a fourth encrypted value to a second server thereby enabling the second server to compute the second database user identifier from an encrypted blinded database user identifier and a decrypted fourth encrypted value; a hardware processor implemented generating a blinded encrypted expression comprising the first database identifier after receiving the first encrypted user identifier; and a hardware processor implemented distributing the first database identifier in a blinded, encrypted form using thereby the first encrypted user identifier, wherein the second encrypted user identifier is generated using an encryption function using a public key of a central authority and a user identity as input and wherein the second database user identifier is generated using a pseudo random function using as input a user identity and the second database identifier, wherein the first encrypted user identifier, the second encrypted user identifier, the first database user identifier and the second database user identifier are deleted after the distribution to the first database and the distribution to the second database, respectively.

7. The system of claim 6 , wherein the first encrypted user identifier is generated using an encryption function using a public key of a central authority and a user identity as input.

8. The system of claim 6 , wherein the first database user identifier is generated using a pseudo random function using as input a user identity and the first database identifier.

9. The system of claim 6 , wherein the distributing to the first database is performed by the central authority and wherein the first database is run on a first server.

10. The system of claim 6 , wherein the distributing to the second database is performed by the central authority and wherein the second database is run on a second server.

11. A non-transitory, computer readable storage medium having instructions stored thereon that, when executed by a computer, implement a method for managing unlinkable database user identifiers, the method comprising: distributing to a first database a first encrypted user identifier, a first database identifier, and a first database user identifier; distributing to a second database a second encrypted user identifier, a second database identifier, and a second database user identifier; receiving from the first database a third encryption and a fourth encryption, the third encryption being formed from the first encrypted user identifier, the second database identifier, and a message comprised in the fourth encryption; decrypting the third encryption thereby obtaining a decrypted value; deriving a blinded user identifier from the decrypted value; sending an encrypted blinded user identifier and a fourth encrypted value to a second server thereby enabling the second server to compute the second database user identifier from an encrypted blinded database user identifier and a decrypted fourth encrypted value; generating a blinded encrypted expression comprising the first database identifier after receiving the first encrypted user identifier; and distributing the first database identifier in a blinded, encrypted form using thereby the first encrypted user identifier, wherein the second encrypted user identifier is generated using an encryption function using a public key of a central authority and a user identity as input and wherein the second database user identifier is generated using a pseudo random function using as input a user identity and the second database identifier, wherein the first encrypted user identifier, the second encrypted user identifier, the first database user identifier and the second database user identifier are deleted after the distribution to the first database and the distribution to the second database, respectively.

12. The storage medium of claim 11 , wherein the first encrypted user identifier is generated using an encryption function using a public key of a central authority and a user identity as input.

13. The storage medium of claim 11 , wherein the first database user identifier is generated using a pseudo random function using as input a user identity and the first database identifier.